CHAPTER 9

Build Better Muscles

Every move you make, from a simple twitch of your nose to an explosive high jump, is powered by your muscles. Regular, challenging exercise builds your muscle strength, which enables your body to work more efficiently. When you train your muscles, you also train your nerve cells to signal your muscle fibers to contract at the same time, which generates force.

PART 1 What Makes a Muscle?

The muscles that connect to your bones and power your movements are called skeletal muscles. They are composed of long, thin cells bound to form a muscle fiber. Fibers are grouped in bundles, which are grouped into larger and larger bundles to form the body of the muscle. Muscle cells are 90% protein. Two key proteins are actin and myosin, called “contractile proteins” because they interact when stimulated by nerve impulses and cause muscle fibers to contract. READ MORE

Each muscle cell is composed of many units of contractile proteins called sarcomeres. Bands of actin and myosin, and some other proteins, are framed by a dividing line called the Z line. When a nerve stimulates the muscle cell, calcium is released in the cell, which triggers the contraction. The actin and myosin fibers move past each other, drawing the Z lines closer together, narrowing the sarcomere unit. This action is powered by adenosine triphosphate (ATP), and generates the force needed for physical exertion. Muscle cells need a lot of energy, so they contain multiple mitochondria, which are also known as cells' “powerhouse” for their energy creating function. After a contraction, the proteins move back and the sarcomere expands again.

Distinct types of muscle tissue power different actions. Slow-twitch, or Type I, muscle fibers have more blood vessels and mitochondria. Endurance activities use a larger percentage of slow-twitch muscle. Fast-twitch, or Type II, muscle fibers are lighter in appearance, and are more heavily relied upon for short bursts of anaerobic activity. Heredity is responsible for the initial proportion of slow-twitch to fast-twitch muscle in any person's body. The main differences between these types of muscle tissue, put simply, is the way that they use energy to create force. Slow-twitch muscle uses oxygen to fuel aerobic activity. Fast-twitch muscle makes and burns fuel mostly without using oxygen. LESS

PART 2 Amazing Satellite Cells

Just the Facts

How dangerous is a muscle tear?

When we exercise at a challenging level, we feel it in our muscles. That slightly tender feeling the day after a workout is a sign that muscles worked hard enough to sustain small tears in the fibers. While the body is at rest—and that rest is vital to building muscle—the muscle fibers are repaired and reinforced with the help of satellite cells, also known as muscles' stem cells. READ MORE

Muscle cells sustain slight damage during resistance training. That's the first step in their becoming stronger. When muscle fibers are damaged, satellite cells leave their usual parking spot on the outside of a muscle cell membrane and travel to the damaged area. These amazing, multitasking cells can multiply and differentiate to rebuild the muscle cell. They can also join together and fuse to the muscle cell, becoming part of the rebuilt, larger muscle fiber. These cells then help the muscle fibers make more contractile proteins. Other satellite cells remain outside the muscle cells, where they can be called into action when another repair job comes along. LESS

PART 3 Other Players in the Repair Process

The process of stimulating satellite cells to build bigger, stronger muscles is controlled by several growth factors. These factors are hormones and chemicals that call the shots when muscles need repair. The capacity to build strength also relies on changes to nerve cells that stimulate the muscles, and some inherited factors. READ MORE

Hepatocyte growth factor (HGF) seems to be responsible for summoning satellite cells to damaged areas. Fibroblast growth factor (FGF) plays a role in the formation of new blood capillaries around the repaired tissue. Two insulin-like growth factors (IGFs) control the amount of muscle growth and new protein formation. And insulin helps glucose get into the cells to fuel the entire repair process. Growth hormone helps the body turn fat into energy that the muscles can use for repair and helps muscle cells gather amino acids and assemble them into new proteins. Finally, the hormone testosterone plays a big role. It can stimulate growth hormone, and increase tissue growth.

Another development in strength training happens in the nervous system. Nerve signals trigger a muscle's response. In a trained muscle, many cells fire simultaneously to create a greater force. This coordination between nerve and muscle cells increases strength even in those who build only modest amounts of muscle. Genetic and gender differences come into play, too. Individuals have different capacities for building and strengthening their muscles. LESS